This invention relates to a nuclear reactor of the type in which fissionable material is placed in the reactor core as fuel in such a way as to provide most effective and economical utilization of the material.
In general, operation of a nuclear reactor is carried out by controlling the excess reactivity of the fuel which is composed of fissionable materials loaded in the reactor core, by a neutron absorber mounted in the reactor. When the excess reactivity is reduced to zero, the reactor is temporarily shut down to perform refueling, that is, the used up fuel in the reactor core is removed and fresh fuel is charged. If need be, shuffling or rearranging of the used and fresh fuels may be performed in the reactor core to effectuate flattening of the power distribution.
Generally, it is of essential requirements for the most economical operation of a nuclear reactor, supposing a certain required fuel enrichment was given, to realize, firstly, optimization of the refueling schedule and, secondarily, optimization of controlling by use of control material, so as to minimize the amount of fuel required. Needless to say, the schemes for these two types of optimization are closely related to each other and cannot be treated separately from each other. So far, however, there is known no theoretically established method of realizing such optimizations to any satisfactory extent.
Heretofore, there has been popularly employed a uniform scatter refueling method which places emphasis on flattening of power distribution and in which, for achieving optimization of the refueling schedule, a fixed quantity of fuel (for example, 1/4 of the entire amount of fuel) in the reactor core is withdrawn successively, beginning with the portion with higher extent of burn-up or use, and a corresponding quantity of fresh fuel is charged and scattered uniformly in the reactor core. In a simplest example of such system, the reactor core is divided into many units so that each unit consists of four fuel assemblies, and refueling is performed by discharging the fuel in each unit successively, beginning with the oldest fuel.
This refueling method features a simple refueling principle and minimized tendency of causing local power peaking as the fresh and old fuels are arranged alternately.
If refueling operation is continued according to such method, there is produced a situation in which burn-up or use of fuel is lowered proportionally to the distance from the center and hence the infinite multiplication factor becomes higher as the distance from the center increases. This is because the power distribution in the reactor core generally has a tendency to decline as the distance from the center increases. This promotes flattening of power as well as temperature distributions of fuel, and consequently, certain thermal allowance is provided.
However, such determined intra-reactor nuclear properties (such as infinite multiplication factor, material buckling, etc., which define the fission productivity) are not necessarily advantageous from the viewpoint of economical utilization (combustion or useful life) of the nuclear fuel.
Also, for performing initial fuel loading, there is employed either a method in which fuel of uniform enrichment is loaded evenly or a method in which, for ensuring flattening of power distribution, a fuel mixture of two to three different degrees of enrichment is loaded such that the fuel of higher enrichment is positioned on the outside. But, either of such methods proves unsatisfactory in always providing effective utilization of nuclear fuel.
On the other hand, for achieving optimization of control by the control material, there is generally employed a method in which a control rod pattern that will maintain power distribution constant during the operation is decided through a trial and error policy by controlling the reactor core uniformly with a liquid poison (such as aqueous solution of boron) or controlling the reactivity by a control rod.
In the study of the problems concerning the above-mentioned two types of optimization, controlling by control material may be regarded as a sub-problem concomitant to the refueling schedule, and it is hence considered that the solution of the latter problem will provide several times as greater effect of optimization than the solution of the former problem.